Check valves of a duckbill configuration are relatively well known and used in the art of valves. The patent literature has a large number of patents disclosing duckbill valves including U.S. Pat. Nos. 3,901,272, 4,524,805, 3,822,720, 4,240,630 and 6,089,260. These patents similarly disclose a valve of a duckbill form having a slit at its outlet. The slit is designed to elastically deform and open when tension is applied about is periphery whereas closure of the slit is automatically provided by biasing stresses in the valve as a consequence of its shape.
The prior art of U.S. Pat. No. 996,588 and German patent no. 4,033,818 describe variants of the duckbill check valves of the preceding art. Both U.S. Pat. No. 996,588 and DE 4,033,818 are valves of a generally conical shape designed to permit flow in a single direction only. U.S. Pat. No. 996,588 is a check valve with a transverse slit through a relatively thick apex portion of the valve which is tensioned under fluid pressure to elastically deform and open. DE 4,033,818 is a pressure relief valve having a discharge aperture at its apex which opens and releases pressure at a predetermined pressure. The valve of DE 4,033,818 is constructed of a highly elastic synthetic resin or rubber which is biased closed but under pressure is stressed about the discharge aperture which is opened.
The applicants (or their predecessors) international patent application no. PCT/AU00/00659 discloses a non-return valve having a valve diaphragm of a conical-shape. The valve diaphragm which is constructed of a resiliently flexible material includes a collapsible aperture which is exposed so as to open under fluid pressure on an upstream side of the valve. The valve diaphragm is tensioned or stressed about the collapsible aperture and the wall thickness of the diaphragm is reduced toward its apex to facilitate this opening of the valve.
These check or non-return valves suffer from at least the following problems:    (i) the differential pressure required across the valve to effect its opening is relatively high;    (ii) the valve may be designed to reduce this differential pressure for opening but then is susceptible to opening and leakage at relatively low differential pressures; and    (iii) the valve in its open condition does not provide great flow throughputs as the throat restriction of the slit or collapsible opening is relatively high.
The applicant's international patent application no. PCT/AU2005/001762 is an example of an attempt to overcome these problems.